Communication systems are well known and consist of many types including land mobile radio, cellular radiotelephone, personal communication system (PCS), and other communication system types. Within the communication system, transmissions are conducted between a transmitting entity and a receiving entity over a communication resource, commonly referred to as a communication channel. To date, the transmissions have typically consisted of voice signals. More recently, however, it has been proposed to carry other forms of data signals. For ease of operation, it is preferable to have the data transmission capability overlay the existing voice communication capability, such that its operation is essentially transparent to the voice communication system while still utilizing the communication resources and other infrastructure of the voice communication system.
One such service associated with cellular radiotelephone communication systems is known as cellular digital packet data (CDPD). The processing of digital packet data signals within the CDPD system is separate from the processing of voice signals in the cellular radiotelephone communication system. However, the CDPD system is integrated with the cellular infrastructure to utilize its voice channels for transmission of digital packet data signals.
Because the CDPD equipment is overlaid on the cellular infrastructure and its operation is essentially transparent to the cellular system, there is no coordination between the two. Thus, the CDPD system must be capable of identifying an unused voice channel on which to transmit so that it does not disrupt voice communications within the cellular system. However, even if the voice channel is unoccupied at the time transmission begins, it is possible that the cellular infrastructure will assign a voice user to the same traffic channel. In this event, the CDPD equipment must quickly detect the presence of the voice user, terminate its transmission and seek out a new unoccupied voice channel on which to resume transmission, i.e., hop from the current voice channel to a new voice channel.
To avoid interfering with the voice user, and degrading the quality of the voice transmission, the hopping capability of the CDPD equipment must respond quickly to the presence of a voice user. That is, it typically must identify the presence of voice signals keying up on the voice channel in approximately 40 milliseconds (msec) or less and hop from the current voice channel to an alternate voice channel. However, the voice signal may be much lower in power than the CDPD digital packet data signal occupying the same channel making detection very difficult. Existing devices, such as scan receivers and subscriber receivers, are disadvantaged by an inability to detect the voice signals, which appear as an interfering co-channel signal, when the voice signal is much lower in power than the existing digital packet data signal. And, even if they were capable of detecting the voice signal, their relatively slow operation would not provide the necessary hopping performance.
Therefore, a need exists for a method and apparatus for hopping digital packet data signals on voice channels of a communication system that quickly identifies the presence of voice signals on the voice channel and selects an alternate communication channel for transmitting the digital packet data signals.